Engine

Dyno Testing

My 86 with 151,000 miles was run on Autothority's rear wheel dyno in PCA Potomac's Dyno for Charity on Saturday. Results were:

188 HP at 5,600 RPM
217 Ft Lbs Torque at 4,500 RPM

This was actually higher than a 87 testest just before my car although it was an automatic which according to Autothority is tougher to test accurately.

Al Cowings said my car was strong. What is a rule of thumb for rear wheel horsepower vs engine horsepower?

Pete Loedding
1986 928 S 5 speed, Black/Black


Pete,
Use a "divide by .84" for a pretty good approximation of the flywheel HP.
188 was what my tired 84 S started at with only headers and no cat. (probably 170 stock to the ground to start) Now Im up to 250 to the ground , with the euro intake and fuel regs , race exhaust, cams, and the eRAM.

Because my engine spec's at 234 flywheel and yours spec's at 288, I would think that your motor is a little tired or you have some problems. I would do a compression check, then also looking at plug shape as well. Automatic, is definately a efficiency looser as well.

188 translates to about 225hp, and you should be in the 260 to 280 range.

Mark K



That is really low. Our baseline dyno for an 86.5 was about 245hp (286hp crank) on the DynoJet 248e., then after some easy bolt-on mods (custom flow-thru exhaust, chip, rising-rate regulator) we were over 300hp (352hp crank).
I think the accepted percentages through-out the industry are 17% for manuals, and 25% for automatics. You should probably have the engine looked over carefully, as you really should be getting atleast 220-230hp at the wheels.

Marc Reviel


Some chip companies use a water brake dyno setup and is not the same as the industry standard inertial wheel dyno...commonly called DynoJet 248 b,c,e,etc. Water brake dynos are good for tuning at a specific rpm range, but provide poor transitional tuning....inertia dynos are far better at tuning a chip or engine for real life racing or street driving. It is much harder because there is not much time to "set the duty cycle" or map on a inertial dyno, but it can be done. Most race teams and tuners now use inertial dynos to set fuel and ign curves.

Our earlier experiences with a water brake was that they proved unreliable and the results could not be correlated to any other dyno.

I would suggest that anyone within a few hundred miles of a DynoJet go get their car dyno'd to at least establish an independent base line...a locator map can be found at http://www.dynojet.com

DEVEK standardized on using a DynoJet for two simple reasons:
1 dyno to dyno consistency (mechanically)
2 there are enough of them around that the customer can dyno his car, put on the improvement and dyno it again and get similar results to what we publish! No need to trust our dyno results...go get your own!

The dyno process is very important.....we always use a minimum of three back to back runs....not with 5 minutes breaks.....but literally back to back. Just enough time to reset the dyno computer for the next run. Four runs is the best and use the third run as the baseline...engine good and hot, any variations in temperature are removed. These are not usually the highest hp runs, but are "normalized". Then do it again with the improvement...and use the third run as the new baseline and compare.

To really check out an improvement, you must be apples to apples......for example, if you want to determine the benefit of a RMB or chip, run your 928 and get a baseline, then run it with ONLY the improvement and compare. Then add wires, plugs, coils, etc. And run it again!

Granted, folks can "cheat" even a dynojet by adding temperature, altitude, etc.....I have even seen folks use 90 % throttle for the "before" tests and 100% throttle for the after tests......!! Or running the before test real hot...then running the after test real cool! There are plenty of ways to cheat any system. But you really only cheat yourself or your customers...

S3's are generally in the 245-260 hp on the ground as measured on a dynojet 248e, and we have seen standard rebuilds register over 280 hp! Of all the engines, the S3 seems most conservative on factory ratings.

Most S4's register 260-288 on the dynojet, some more, some less. We have dyno'd more S4's than I care to think about...and on two different dynos...they are always in the same range.

GTS range from 285-310 on the ground.

Changing spark plugs from an old set....1K or more miles, to a new set will add 3 - 7 hp! but the gain will not last for long. In comparison of a new plug against a different new plug 0 HP difference!

New wires - stock- will add another 6 - 10 hp if the old ones are out of spec or arcing.

Autos 5-10% less, due to pumping losses of the auto tranny.

A hp conversion factor "rule of thumb" MIGHT be .85 to .88 x crank hp...but remember...these engines had a standard variance of +/- 4% right from the factory! And there were some 104% + made for some folks. I know 1 GTS ordered and built that way! These numbers assume that the engine is in excellent mechanical condition.....not all are.

Hope this helps,
Marc Thomas
DEVEK
http://www.devek.net